This fact is easy to take for granted, yet it’s the reason we can surf the web using a wireless router located in another room.

But not all of that microwave radiation makes it to (or from) our phones, tablets, and laptops. Routers scatter and bounce their signal off objects, illuminating our homes and offices like invisible light bulbs.

Now, German scientists have found a way to exploit this property to take holograms, or 3D photographs, of objects inside a room — from outside it.

“It can basically scan a room with someone’s Wi-Fi transmission,” Philipp Holl, a 23-year-old undergraduate physics student at the Technical University of Munich, told Business Insider.